This invention relates to the smoothing or polishing of surfaces of crystalline materials such as sapphire and the like to remove microscopic imperfections and more particularly to a process for producing nearly atomically smooth surfaces thereon by removing material from such surface substantially atomically at a given rate and simultaneously depositing identical material on such surface substantially atomically at an independently controlled rate closely approaching and not greater than the given rate of removal.
In the fabrication of a variety of optical and electronic devices bodies of sapphire or the like having at least one flat surface which is as smooth as possible are required. Dielectric substrates for semiconductor devices are one example of such requirements.
Mechanical polishing techniques have been used in the prior art which, with careful attention to detail, are capable of producing surface finishes having no scratches or other surface imperfections visible at magnifications of 400.times.. Further improvement in surface polishing by mechanical techniques can only be obtained at great expense in terms of time as well as money and where the body to be polished is made of soft material or material which is hygroscopic, mechanical polishing techniques present further problems.
Thus, in the prior art, a number of chemical polishing techniques have been developed based on the use of liquid, vapor or gaseous etchants. Such techniques have included the steps of heating the body to an elevated temperature below the melting point thereof and then subjecting the surface to be polished to a flowing etchant in an attempt to remove the surface imperfections. However, all of such proposals have the disadvantage that it is difficult to accurately control the etch rate over the surface of the body. Perfectly homogenous mixtures of fluids are difficult, if not impossible, to obtain and maintain and a homogenous flow of gases over the surface to be polished is impossible where the surface has a complex shape and difficult even on flat surfaces due to the very presence of the surface defects which are to be removed. Furthermore, different crystal faces will be etched at different rates resulting in the introduction of surface imperfections due to the etching process itself where different crystal faces are exposed as in the case of single crystal bodies having complex surface shapes or polycrystalline bodies. Thus, etching techniques heretofore proposed have been time consuming and expensive and have tended to introduce surface imperfections or to reinforce the surface imperfections already present, particularly where such imperfections are severe.
U.S. patent application Ser. No. 610,156 (now U.S. Pat. No. 4,038,117) mentioned hereinabove teaches a method of surface polishing bodies of sapphire and the like by heating such body to a temperature of about 1900.degree. C. in a substantially static hydrogen gas environment. At such temperature, a reversible reaction occurs between the alumina and the hydrogen resulting in alumina being simultaneously removed from the surface and redeposited thereon. The result is a superior polishing action providing surfaces having no imperfections visible at magnifications of 1000.times. and more after one hour of treatment. However, the process is basically an equilibrium process in which a given etching or vaporization rate and a given redeposition rate is established at a given temperature. Thus, it is impossible to vary the redeposition rate without varying the etching or vaporization rate and vice versa.